In my co-pending application Ser. No. 883,289, filed July 7, 1986, as a Continuation-In-Part of application Ser. No. 767,949, filed Aug. 21, 1985, I describe an optical proximity sensor which is useful in determining the distance to a surface or an object without actually touching the surface or object itself. Such devices are useful in process controls as well as in photography to determine the distance to a remote object.
In one embodiment of the above-mentioned co-pending application, two energy emitting sources such as light emitting diodes are mounted to project light onto the remote surface from two slightly different distances away therefrom. Energy reflected from the surface is detected to produce a resultant output signal in accordance with the flux received. In this case, the two energy transmitters are modulated at a different phase angle and it is shown therein that the phase of the output signal produced by the detector is related to the distance to the remote surface. In a second embodiment a single energy emitting source is utilized to direct energy to the remote surface and two detectors situated at different distances from the remote surface receive the reflected energy. The output of one of these detectors is phase shifted with respect to the other to produce a resultant output signal and again it is shown that the phase of the output is related to the distance to the surface. One advantage of the second embodiment is that the light emitting diodes are temperature sensitive and may not track well with changes in temperature when more than one LED is employed whereas, with a single LED, the variation in temperature does not affect the accuracy of the system.
Both of the embodiments utilize phase shifters and synchronous detectors which may add some undesirable complexity and cost to a system.